1. Technical Field
The present invention relates to fuel cells, and particularly to monitoring and control systems for fuel cells.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are serially coupled electrically to form a fuel cell stack having a desired power output.
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of reaction products, such as water, formed during operation of the cell.
Due to their zero- or low-emission nature, and ability to operate using renewable fuels, the use of fuel cells as primary and/or backup power supplies is likely to become increasingly prevalent. For example, a fuel cell stack can serve as an uninterruptible power supply for computer, medical, or refrigeration equipment in a home, office, or commercial environment. Other uses are of course possible. Operating and environmental factors relevant to efficient fuel cell system operation may include the concentration of hydrogen in the surrounding environment, the concentration of oxygen in the surrounding environment, fuel cell stack temperature, ambient air temperature, current flow through the fuel cell stack, voltage across the fuel cell stack, and voltage across the MEAs. These factors become increasingly relevant when the fuel cell operating environment is a human habitable space with a low air flow exchange rate and/or when the space is small, such as a utility room or closet.
Consequently, there is a need for improved control systems for fuel cell systems, particularly for fuel cell systems that operate in enclosed environments and/or habitable environments, and for methods of controlling such fuel cell systems.
According to one aspect of the invention, there is provided an electrochemical power generation system that includes a fuel cell stack including at least one fuel cell, an oxidant inlet, an oxidant outlet, a fuel inlet and a fuel outlet, a fuel delivery system for delivering fuel to the fuel inlet of the stack, an oxidant delivery system for delivering air from the ambient environment to the oxidant inlet of the stack, and an oxygen sensor for measuring the oxygen concentration of ambient air in the vicinity of the power generation system. A controller is coupled to the oxygen sensor and configured to cease operation of the power generation system when the oxygen concentration of the ambient air in the vicinity of the power generation system falls below an oxygen concentration threshold. The power generation system is particularly suitable for operation inside a habitable, confined space, such as a small room or a closet.
Ceasing operation of the power generation system, as used herein, means stopping power-producing operation of the fuel cell stack, and does not necessarily include ceasing operation of various components of the power generation system, such as the controller, sensors, etc., which may be powered by an alternative power source such as a battery after the stack stops producing power.
The electrochemical power generation system may also include a purge valve that is associated with the fuel outlet. The controller may be coupled to the purge valve and be configured to intermittently open the purge valve such that the hydrogen discharged from the fuel cell stack during operation of the power generation system does not cause the hydrogen concentration in the vicinity of the power generation system to exceed a high hydrogen concentration condition before the oxygen concentration in the vicinity of the power generation system falls below the oxygen concentration threshold. In particular, the controller may be configured to intermittently open the purge valve such that the average continuous rate of hydrogen discharged from the fuel cell stack during operation of the power generation system does not exceed a critical hydrogen discharge rate that would cause the hydrogen concentration in the vicinity of the power generation system to exceed a high hydrogen concentration condition before the oxygen concentration in the vicinity of the power generation system falls below the oxygen concentration threshold.
The electrochemical power generation system may further include a hydrogen concentration sensor that measures the hydrogen concentration in the ambient air in the vicinity of the power generation system. The controller may be coupled to the hydrogen concentration sensor and be configured to cease operation of the power generation system when the hydrogen concentration measured by the hydrogen concentration sensor exceeds a hydrogen concentration threshold. The controller may, for example, be configured to close the purge valve when the hydrogen concentration measured by the hydrogen concentration sensor exceeds the hydrogen concentration threshold. The hydrogen concentration threshold may be suitably set at 1%, and the oxygen concentration threshold set at 18%, for example. The high hydrogen concentration condition may be set to correspond to a lower flammability limit of hydrogen, which is typically approximately 4% of atmosphere.
The electrochemical power generation system may further include a temperature sensor, in which case the controller may be configured to cease operation of the power generation system in response to a temperature reading that exceeds a high temperature threshold.
According to another aspect of the invention, there is provided a method of operating a fuel cell electrochemical power generation system that includes the steps of directing fuel to a fuel cell stack, directing air from the ambient environment to the fuel cell stack for use as oxidant, monitoring the oxygen concentration of the ambient air in the vicinity of the power generation system, and ceasing operation of the power generation system if the monitored oxygen concentration falls below an oxygen concentration threshold. This method is particularly suitable when the power generation system is operated inside a habitable, confined space, such as a small room or a closet.
This method may include the further step of intermittently discharging hydrogen from the fuel cell stack in a manner that does not cause the hydrogen concentration in the vicinity of the power generation system to exceed a high hydrogen concentration condition before the oxygen concentration in the vicinity of the power generation system falls below the oxygen concentration threshold value. In particular, hydrogen may be intermittently discharged from the fuel cell stack such that the average rate of hydrogen continuously discharged does not exceed a critical hydrogen discharge rate that would cause the hydrogen concentration in the vicinity of the power generation system to exceed a high hydrogen concentration condition before the oxygen concentration in the vicinity of the power generation system falls below the oxygen concentration threshold value.
The method may further include monitoring the hydrogen concentration in the vicinity of the power generation system, and ceasing operation of the power generation system if the hydrogen concentration exceeds a hydrogen concentration threshold. The hydrogen concentration threshold may be set at approximately 1%, and the oxygen concentration threshold set at approximately 18%, for example. The high hydrogen concentration condition may be set to correspond to a lower flammability limit of hydrogen, which is typically about 4% of atmosphere.
The method may further comprise monitoring the temperature of the power generation system, and stopping operation of the power generation system in response to a temperature reading exceeding a high temperature threshold.
According to yet another aspect of the invention, there is provided a computer-readable media that contains instructions to cause a controller to control operation of a fuel cell stack by monitoring the oxygen concentration in ambient air in the vicinity of the fuel cell stack during operation of the fuel cell stack, and by ceasing operation of the fuel cell stack if the concentration of oxygen of ambient air in the vicinity of the fuel cell stack is less than an oxygen concentration threshold. The computer-readable media may comprise a memory structure of a micro-controller.
The computer-readable media may also contain instructions for the controller to limit the average rate of hydrogen continuously discharged from the fuel cell stack to not exceed a critical hydrogen discharge rate that would cause the hydrogen concentration in the vicinity of the power generation system to exceed a high hydrogen concentration condition before the oxygen concentration in the vicinity of the power generation system falls below the oxygen concentration threshold setting. The oxygen concentration threshold may be suitably set at approximately 18%. The controller may be instructed to monitor the oxygen concentration periodically or continuously. The high hydrogen concentration condition may be set to correspond to a lower flammability limit of hydrogen, which is typically approximately 4% of atmosphere.